The project will examine the role of the suprachiasmatic nucleus (SCN) in age-related changes in circadian rhythms. In both humans and other animals, the regulation of circadian rhythms changes with age. Most notably, the free-running period shortens and the amplitude of circadian rhythms decreases. In humans, a shortening of free-running period may be cause if the tendency for early awakening in older individuals. The SCN is a cluster of cells within the hypothalamus that generates circadian oscillations and plays a central role in the regulation of circadian rhythms. It is likely that aged-related changes within the SCN underlie the changes in circadian rhythms. The project will use neural transplantation techniques in Syrian hamsters (Mesocricetus auratus) to test several critical hypotheses about the relationship between age-related changes in the SCN an in behavioral rhythms. Surgical ablation of the SCN will be used to disrupt the circadian activity/rest rhythm, and transplantation of a fetal SCN will be used to restore the rhythms. SNCs will be transplanted into young and old hamsters to determine whether it is the age of the SCN rather than the age if the whole animal that is critical for age-related changes in properties such as free-running period. The hypotheses that shortening of free-running period is due to a loss of SCN tissue. To test the hypotheses that the ability of the young, grafted SCN to interfere with the intact SCN of the host will be measured. These experiments will provide critical evidence for whether or not the SCN is the locus of age-related changes in circadian rhythms. Such evidence is essential to further understand neural basis of age-related changes in behavior.